The present invention relates to an optical disk apparatus for recording and/or reproducing data to or from an optical disk having information record tracks.
In general, the optical disk apparatus uses a coarse actuator for performing coarse positioning and a fine actuator for performing fine positioning. These actuators cause an optical beam spot to follow the target track on the optical disk.
In the conventional optical disk apparatus, only a fine actuator is driven to perform a track jump and move an optical beam spot to the next track. The fine actuator used may be one of several different types. For example, the fine actuator may be a lens actuator type for moving a lens itself in a tracking direction, or a tracking mirror type for swinging an optical beam to position an optical beam spot.
When using either the lens actuator type or the tracking mirror type actuator, the tracking control is performed on the basis of a tracking error signal. FIG. 1 shows a method for detecting the tracking error signal. In this method, light beam 5 involving a tracking error is incident on tracking detection light-receiving elements 7a and 7b constituting a divided photodetector 7. The light beam 5 is converted into electric signals indicative of the amount of light. The electric signals are subtracted in a differential amplifier 8, to obtain the tracking error signal. The track jump operation is performed during tracking control in response to a track jump instruction.
During usual tracking control, a compensation signal is generated by a tracking compensation circuit 10 on the basis of the tracking error signal from the differential amplifier 8, as is shown in FIG. 2. This compensation signal is supplied to a fine actuator driving control circuit 24 via a switch 22, to thereby control a fine actuator 26. At the time of the track jump operation, a positive or negative jump pulse corresponding to a movement direction is generated from a jump pulse generating circuit 20 and the switch 22 is switched, in response to the track jump instruction. As a result, the jump pulse is input to only the fine actuator 26 via the fine actuator driving control circuit 24, thereby accelerating or decelerating the actuator 26 to execute track jump of the optical beam spot.
Concerning the tracking error signal necessary to tracking control, it is known that an offset occurs in the signal when the fine actuator 26 is displaced. The amount of the offset is proportional to the amount of displacement of the fine actuator. For example, the greater the amount of displacement, the greater the amount of the offset. Thus, the offset in the tracking error signal makes the tracking control unstable.
Specifically, when the fine actuator 26 is displaced a large amount at the time of the track jump operation, an offset corresponding to the amount of displacement will occur in the tracking error signal. Where the tracking mirror type actuator is used as the fine actuator 26, and the tracking mirror of the actuator is driven at the time of the track jump operation, the optical beam spot or optical beam itself is greatly shifted a large amount according to the rotation of the tracking mirror. This causes an offset in a tracking error signal detected by the divided photodetector 7 after reflected from the surface of the optical disk.
On the other hand, where the lens actuator is used as the fine actuator 26, the optical axis of the lens itself is shifted from the line of the optical beam. This shifts the line of an optical beam reflected from the optical disk, causing an offset in the tracking error detection system. Although the manner of occurrence of an offset in the tracking error signal and the rate of the offset differ between the two types of actuators, the influence of the offset upon tracking is conspicuous at the time of the track jump operation in which the fine actuator 26 is greatly shifted.
As described above, in the optical disk apparatus, an offset inevitably occurs in the tracking error signal at the time of the track jump operation. Therefore, the tracking control itself is very unstable, making it difficult to finely converge onto a target track.
While more strict specifications are required for recording and/or reproducing data with a reduction in the interval between tracks made to realize high density recording, the effect of the inclined surface of an optical disk relative to the line of an optical beam may increase when the optical disks clamped and rotated, thereby degrading read/write data signals. Further, it is known that the inclination not only degrades the read/write data signals but also causes an offset in the tracking error signal. The offset not only makes the tracking control unstable but also increases the instability of the tracking control when performing the track jump operation.
To keep the quality of signals within a predetermined range, so-called tilt servo control is generally executed in which the inclination amount of the normal line of the optical disk surface relative to the line of the optical beam is detected and reduced to zero. However, in the tilt servo control, high-speed correction is not performed and the inclination is corrected slowly, and accordingly the tracking control is executed for a long time with the inclination amount kept at a relatively high value. In other words, even if the tilt servo control is executed, the amount of inclination may be large momentarily. If the track jump operation is performed at that moment, the tracking control and the track jump control itself will inevitably become unstable.
Furthermore, if an inclination detector incorporated in a circuit for the tilt servo control detects the inclination using a signal depending upon the optical beam spot, for example, using a signal output from the divided photodetector 7, the amount of the inclination detected for the tilt servo control is multiplied by an offset signal due to a jump signal when performing the track jump operation. As a result, the tilt servo control is performed on the basis of the offset signal, which means that an error operation is performed instead of fine tilt servo control.
As described above, at the time of performing the track jump operation, the fine actuator is driven, and the tracking error signal is multiplied by an offset signal resulting from the displacement of the fine actuator. This makes it hard to correctly move an optical beam to a target track, and may well make the tracking control and the track jump control itself unstable. On the other hand, the inclination of the optical beam relative to the surface of the optical disk causes multiplication of the tracking error signal by the offset signal, thereby increasing the instability of the tracking control. Thus, execution of the track jump operation in the state that the line of the optical beam is greatly inclined relative to the normal line of the surface of the optical disk causes the target track not to be reached, and also causes an increase in the instability of the tracking control and track jump control.